Method1. Preheat the oven to 175C (350F).2. In a small saucepan over medium heat, melt the butter.3. Add the brown sugar and stir; cook until the mixture comes to a boil. Reduce the heat and let the mixture simmer for a minute. Then remove the saucepan from the heat, stir the vanilla in, and allow the mixture to cool down for about 10 minutes.4. In a medium bowl, whisk the egg to break up the yolk then whisk a spoonful of the brown sugar mixture in; then add the rest of the brown sugar mixture and stir until combined.5. Add the flour, baking powder and salt and stir gently until combined.6. Line a 20cm square baking tin with baking paper and pour the batter in.7. Bake for 25-30 minutes, until the edges look slightly browned and crispy.8. While still warm, cut into squares. When they cool down more, the chewiness gets much more intense!